Disable copy-code button for mermaid diagrams

Related issues - #349497 (closed), #349892 (closed)

What does this MR do and why?

It removes the copy-code button for mermaid diagrams, fixing #349497 (closed), and potentially #349892 (closed) also.

It was added in !75433 (merged), and intended for only code blocks and not the mermaid diagrams I suppose.

Screenshots or screen recordings

1. Copy clipboard button

before	after (renders only for code block)

2. Mermaid diagram rendering issue

Intermittent issue (both diagrams are not rendered)	Intermittent issue (one diagram is not rendered)	After fix

How to set up and validate locally

1. Create a new issue / MR with the following comment
2. Enclose it with mermaid ...

sequenceDiagram
    User->>GitLab: GET /project/-/archive/master.zip
    GitLab->>User: 302 Found
    Note over User,GitLab: Location: https://cdn.com/project/-/archive/master.zip?token=secure-user-token
    User->>CDN: GET /project/-/archive/master.zip?token=secure-user-token
    alt object not in cache
      CDN->>GitLab: GET /project/-/archive/master.zip
      Note over CDN,GitLab: X-Gitlab-External-Storage-Token: secure-cdn-token<br/>X-Gitlab-Static-Object-Token: secure-user-token
      GitLab->>CDN: 200 OK
      CDN->>User: master.zip
    else object in cache
      CDN->>GitLab: GET /project/-/archive/master.zip
      Note over CDN,GitLab: X-Gitlab-External-Storage-Token: secure-cdn-token<br/>X-Gitlab-Static-Object-Token: secure-user-token<br/>If-None-Match: etag-value
      GitLab->>CDN: 304 Not Modified
      CDN->>User: master.zip
    end

graph LR; A-->C;

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.